In the process of drug discovery, quantitative characterization of inhibitory potencies of all the potential drug candidates is often needed to figure out the best candidate. For comparing potential drug candidates, an outcome of curve fitting of a sigmoidal curve to the dose response data or the value of half maximal inhibitory concentration, IC50, is able to be used. Inhibitors are small molecules that impede the substrate molecules from reaching or binding to the enzyme active sites. Half of today's drugs are inhibitors. In dose response analysis, the use of a wide concentration range is important because the dose response is represented by a Maltusian-type differential equation. The solution of the equation is logarithmic type. Recently, the importance of a log scale for drug response analysis, based on thermodynamical basis, has been reported. The drug response is directly proportional to the log of the drug concentration, which is expressed as μ=RT ln[a], where μ is chemical potential or response of the drug, [a] is an activity or concentration of the drug, R is a constant number, and T is a temperature (e.g. absolute temperature). Hence, drug concentrations that are equally spaced using a logarithmic scale instead of a linear scale are selected.
Conventionally, serial dilutions with test tubes, vials, and micro-plates are routinely used for the preparation of a concentration gradient in the determination of IC50. However, it is well known that the serial dilution method introduces ample errors through pipetting that directly affect the precision of each dilution ratio. The error in each step of serial dilution is able to lead to significant errors after several steps of serial dilution. This is able to affect the evaluation of IC50 values of drug candidates and their realistic comparisons. Therefore, direct dilution of drugs has been recommended for the evaluation of IC50.
Conventional approaches are available for handling samples with direct dilution by using individually addressable piezo dispenser and acoustic droplet ejection through a vibrating piezo transducer. These involve the use of conventional micro plates for hosting reagents and letting the reactions to occur. These conventional methods still need milliliters or liters of reaction solutions.